Maurizio Licchelli

The design of luminescent sensors for anions and ionisable analytes

Abstract A molecular luminescent sensor for anions can be built through a modular approach, i.e. by covalently linking an appropriate photoactive fragment to the receptor displaying a satisfactory affinity towards the desired substrate. Following the receptor-anion interaction, an intercomponent process must take place, e.g. an electron transfer (eT) or an energy transfer (ET) process, that distinctly modifies the emission of the luminophore, thus signalling the occurrence of the recognition event. In this article, specific molecular sensors are classified according to the type of receptor-anion interaction, whether hydrogen bonding or metal–ligand interactions. Receptors of the latter class are based on a Zn II polyamine platform, which leaves at least a vacant coordination site for the incoming anion. Substrates include natural amino acids, NH 3 + CH( R )COO − , for which the highest selectivity is observed when the receptor subunit specifically interacts with the R portion. An eT process involving R and the nearby excited luminophore may provide the signal transduction mechanism.

Urea vs. thiourea in anion recognition

Neutral anion receptors (LH) form stable 1 : 1 H-bond [LH...X]- complexes with carboxylates, halides and phosphate (X-). Some of the [LH...X]- complexes, in presence of an excess of X-, release an HX fragment, with formation of [HX2]- and the deprotonated receptor L-. The tendency towards deprotonation increases with the acidity of the receptor and with the stability of the [HX2]- self-complex. Thus, the more acidic thiourea containing receptor deprotonates in the presence all the investigated anions except chloride, whereas the less acidic urea containing receptor undergoes deprotonation only in the presence of fluoride, due to the high stability of [HF2]-.

Molecular switches of fluorescence operating through metal centred redox couples

Three-component molecular systems (redox active subunit)-spacer-(light-emitting fragment) can operate as fluorescence switches, following the alternate addition of an oxidizing agent and a reducing agent (or the adjustment of the potential of the working electrode in an electrolysis experiment). The redox active subunit typically consists of a metal centred redox couple (M(n+1)+/Mn+), encircled by a macrocyclic receptor, and switching efficiency requires that one of the two oxidation states quenches the proximate fluorophore and the other does not. Four ON/OFF systems, based on either the CuII/CuI or NiIII/NiII couple, will be discussed. The nature of the quenching process responsible for the OFF state, either electron transfer or energy transfer, is related to the length and to the flexibility-rigidity of the spacer.

A two-channel molecular dosimeter for the optical detection of copper(II)

A cyclam-like macrocycle with an integrated push-pull chromophore selectively detects Cu2+ inclusion through both orange-to-yellow colour change and quenching of the green fluorescence.

Sensing of transition metals through fluorescence quenching or enhancement. A review

A series of fluorescent sensors for transition metal ions were synthesized by linking a light-emitting subunit, anthracene, to a polyaza chelating subunit, either a dioxotetraamine or a tetraamine. Sensing of the divalent cations CuII, NiII and ZnII was investigated through spectrofluorimetric titrations in acetonitrile–N water (4:1) solutions. The selective recognition of CuII and NiII among other transition and non-transition metal ions is signalled through full quenching of fluorescence; discrimination between the two ions can be achieved by performing titrations at controlled pH. The system containing the tetraamine fragment, whose interaction with CuII and NiII induces fluorescence quenching, is also sensitive to ZnII, but in this case the recognition is signalled through a fluorescence enhancement.

Molecular events switched by transition metals

Transition metals can typically give rise to two (or more) distinct states of comparable stability (two consecutive oxidation states; two different stereochemical arrangements). In a multicomponent system, the conversion of one state to the other can modify a given property of a nearby subunit or can induce drastic changes in the system topology. In this sense, the metal behaves as a switch, which can be operated through an external input (the variation of the pH or of the redox potential). Recent examples of molecular switching by transition metals are reviewed: (i) the quenching/enhancement of the emission of a luminescent fragment effected by a nearby metal centred redox couple (e.g. NiII/NiIII); (ii) the pH driven motion of an aminoalkyl side chain in a NiII scorpionate complex, which is signalled by the variation of the light emission intensity of an appended anthracenyl fragment; (iii) the pH controlled translocation of a NiII ion within a multidentate ligand containing two compartments of different coordinating tendencies and (iv) the intramolecular translocation of a Cl− anion between two pre-positioned metal centres CuII and NiII, within a ditopic receptor, electrochemically driven through the NiII/NiIII redox change.

Vinylic Polymerization of Norbornene by Late Transition Metal‐Based Catalysis

The polymerization of norbornene with two new families of late transition metal-based catalysts derived from: (i) Ni and Pd complexes bearing bulky diammine ligands, with the general formula [ArN=C(R)-C(R)=NAr]MeX 2 and (ii) Fe and Co complexes bearing bulky arylimine ligands with the general formula [(2,6-ArN=C(Me)) 2 C 5 H 3 N]MX 2 is reported. New Co-based complexes have been tested as well. A prevailingly vinylic, amorphous polymer was obtained with all the catalysts tested, whose solubility in organic solvents is substantially dependent on the molucular weight. Polymerization activity greatly varies from one catalyst to another and depends rather on the metal than on the ligand. Solvent polarity and temperature greatly affect the polymerization yields. Besides a dramatic reduction of molecular weights, the addition of small amounts of 1-hexene produces a noticeable increase in the catalytic activity. NMR analysis shows that in all cases a certain percentage of ROMP polymer is present and that, in general, the variations in polymerization conditions, which produce an increase in activity, simultaneously affect a reduction of the ROMP percentage.

The design of fluorescent sensors for anions: taking profit from the metal–ligand interaction and exploiting two distinct paradigms

Fluorescent sensors are molecular systems consisting of a receptor moiety and of a fluorogenic fragment, which are capable of recognising a given analyte and signalling recognition through a variation of the emission intensity. The fluorogenic fragment responsible of the signal can be associated to the receptor either covalently or non-covalently, giving rise to two well distinct classes of fluorosensors and sensing paradigms. The design of fluorescent sensors is described, with a special attention to the sensing of anionic groups (including those of amino acids). In any case, it seems convenient that the receptor moiety contains one or more metal centres, which establish strong coordinative interactions with the envisaged anionic substrate. Selectivity is related to the energy of the metal–analyte interaction and can be achieved by taking profit of the concepts developed in more than one hundred years of coordination chemistry. As an example, recognition and sensing of the amino acid histidine is considered in detail, which is based on the attitude of the imidazole residue to deprotonate and bridge two MII ions prepositioned at the right distance, within a defined coordinative framework (M = Cu, Zn).

A FLUORESCENT CHEMOSENSOR FOR THE COPPER(II) ION

Abstract The two-component system anthracene-9-carboxylic acid 1,4,8,11-tetrathia-cyclotetradecan-6-ylester ( 1 ) has been synthesized, structurally characterized and investigated as a fluorescent chemosensor for the Cu 11 ion. Crystallographic details for 1 : space group P 2 1 / c with α = 9.894(2), b = 24.608(2), c = 10.759(2) A , α = 90, β = 105.88, γ = 90°; V = 2519.7 A 3 , Z = 4 (R = 0.088, R w = 0.066) . In ethanolic solution 1 selectively incorporates Cu 11 into its tetrathia crown component in the presence of other 3d metal ions and signals the recognition through the quenching of the fluorescence of the anthracene fragment. Quenching of the photo-excited state (which has a charge transfer nature) takes place through a photo-induced electron transfer from the fluorophore to the metal center and involves the Cu II /Cu I couple. The d 10 cation Ag 1 competes successfully with Cu 11 for the tetrathia cavity of 1 in an ethanolic solution (not in an acetonitrile solution), but the occurrence of the metal/receptor interaction cannot be signalled, due to the poor redox activity of the non-transition cation Ag 1 .

Controllable Intramolecular Motions That Generate Fluorescent Signals for a Metal Scorpionate Complex

Simply by changing the pH value, the side chain of complex 1 can be reversibly moved between two positions. Coordination to the metal center through the nitrogen atom of the side chain at moderate pH values is accompanied by a decrease in fluorescence intensity (from IF =100% to IF =60%). A further decrease is observed upon deprotonation of the bound water molecule at higher pH (IF ≤2%). Therefore, 1 can be seen as a molecular three-position switch.